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University of California. 



[Vol. 1 . 



almost isotropic, exhibiting generally the faint double refraction in 

 the form of stripes, so characteristic of analcite. The mineral 

 proved to be soluble in hydrochloric acid, and the solution upon 

 evaporation to dryness left crystals of sodium chloride. It was 

 somewhat more difficultly fusible than is stated to be the case with 

 analcite in the works upon blowpipe analysis. The specific gravity 

 ascertained by Klein's solution was 2.266. The analcite occurs 

 under four different conditions in the rock, namely: (1) Lining the 

 sides of cavities frequently two centimeters in longest diameter; (2) 

 filling the angular spaces between the divergent feldspar crystals, 

 spaces from microscopic size up to eight millimeters in diameter; 

 (3) replacing the feldspars; and (4) in one of the dikes, in the form 

 of hexagonal or rounded grains partly inclosed within the feldspars. 

 It is possible that the larger cavities about which the analcite is 

 crystallized are of secondary origin, as they occur only locally and 

 in portions of the rock which are most decomposed, but they do 

 not differ materially in character from some of the larger spaces 

 entirely filled with analcite, on which, as in the case of the crystals 

 lining the cavities, striations appear. Faces of the trapezohedron, 

 the common crystallographic form of analcite, were observed on 

 some of the crystals lining these cavities. 



At the termination of the crystallization of the feldspar and 

 augite sharply angular areas were left between the divergent crys- 

 tals, which are now filled with analcite and its decomposition 

 products. To account for the presence of the analcite we have to 

 adopt one of four hypotheses: The possibility of its being primary, 

 of its derivation from the feldspar, of its resulting from the decay 

 of some unknown soda-rich mineral, or of its introduction into the 

 rock subsequent to its solidification. Of the first there is no con- 

 firmatory evidence whatever. With one exception analcite has 

 always been considered of secondary origin. The analcite of the 

 basalt described by Lindgren,* from Montana, is considered by him 

 as primary. This opinion is based on the fresh condition of the 

 rock and the predominance of hexagonal sections among those 



*Erupiive Rocks from Montana. Proc. Cal. Acad. Sci., Vol. III., 1891, 



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